Bilateral breast metastatic rectal cancer with MET gene amplification: A rare case report.

Breast metastases originating from colorectal cancer are extremely rare, accounting for <0.5% of all malignant lesions of the breast and are often associated with histopathological forms with a poor prognosis, such as mucinous or signet-ring cells, in which bilateral breast metastases are even rarer. In this case report, we present a female patient in her early 50s. In April 2023, the patient was diagnosed with cT3N2M0 rectal cancer. After that, the patient underwent Miles surgery, followed by 12 cycles of mFOLFOX6 adjuvant chemotherapy, and received adjuvant chemoradiation therapy (50.4 Gy plus Capecitabine). The patient was discharged in January 2024. However, 7 months later, due to a palpable mass in her breast, the patient was found to have bilateral breast lesions and peritoneal metastases. Biopsies were taken from the lesions in both breasts and immunohistochemistry revealed that the tumors originated from the rectum with genetic testing detecting MET gene amplification. Thereafter, the patient was treated with many regimens, but the disease did not respond and progressed rapidly. The patient died six months after the recurrence of the disease was detected. In conclusion, breast metastases are a poor prognostic feature for colorectal cancer. Patients presenting with malignant breast lesions and a history of non-breast malignancy require careful differential diagnosis.